Black oxide coatings are a known class of finishes used with stainless steel alloys and other ferrous metals for enhancing appearance, corrosion resistance and the like. A variety of treatment techniques have been employed over the years for producing the desired black oxide coatings on the various types and forms of ferrous metals. It has been found, however, that the metal finishing treatments conventionally used with the various grades of stainless steel alloys to produce the desired black oxide coating are not suitable for grades of steel alloys that may be hardened by heat treatment such as 400 series martensitic stainless steel. For example, a fused salt oxidizing treatment for corrosion resistant steel alloys having a draw temperature greater than 900.degree. F. employs a process temperature of 800.degree. F. or higher, which temperatures unduly temper hardened 400 series steel resulting in the loss of physical properties and/or adversely affecting close dimensional tolerances of treated parts, and an alkaline oxidizing process suitable for providing a black coating on forms of 300 series corrosion resistance steel alloys, cast and malleable iron employs a treating bath with oxide/sulfide components which, when used for treatment of 400 series steel alloys, results in products having reduced corrosion resistance.
Additionally, an alkaline oxidizing process heretofore suggested for treating 400 series stainless steel alloys to achieve the desired black oxide coatings, employs an alkaline-chromate-oxidizing treatment bath operated at temperatures of 285.degree. F. or higher, which was found to be unsuitable for producing the desired black oxide finish, or for that matter, a black finish of any type.